The present invention relates to saturators for impregnating a substrate with a saturant, and in particular to an improved saturator and saturating method for impregnating selected portions of a substrate with increased amounts of a saturant.
Saturators have been used for some time to impregnate substrates such as webs of paper with varying amounts of saturants. By properly selecting the amount and type of saturant to provide the desired characteristics to the substrate, saturators can be used to enhance the physical characteristics, and therefore the value, of the substrate.
For example, one valuable saturant is sodium silicate. When high levels of sodium silicate are added to a paper web, the paper can be made fire resistant and can be given much improved structural strength. However, such highly impregnated paper can be difficult to fold or crease in conventional paper processing machines. It therefore would be advantageous to impregnate a paper web with increased amounts of sodium silicate at selected portions of the web. For example, if a paper web were to be used to form a box in which stacking strength were an important consideration, it would often be advantageous to apply increased amounts of sodium silicate to the sidewalls of the box as compared with the top and bottom panels, which must be folded in use.
As another example, containers such as beer cases are subjected to unusual wear patterns. The tops and bottoms of the cans within the case act as cookie cutters during transportation and can severely damage either the printing on or the actual structure of the top and bottom panels of the case. If sodium silicate were applied to the top and bottom panels, this cookie cutter effect could be resisted effectively. In this example, however, there is no need to apply large amounts of sodium silicate to the sidewalls, and it would reduce the cost of materials if the saturant could be concentrated at the top and bottom panels and not the sidewalls of the case.
In spite of the important advantages that selective saturation would provide in the examples described above, the applicant is unaware of any commercially available saturator that performs this function. The saturator described in Menser U.S. Pat. No. 4,588,616 is an extremely effective device which can be used to saturate substrates with a range of saturants at both relatively low and extremely high add-on weights. Similarly, U.S. Pat. No. 2,711,032 describes another type of saturator used in the past. However, neither of these saturators is provided with means for selectively impregnating desired portions of the web with increased amounts of the saturant.
In the past, stencils have been used with a variety of surface applicators for liquids of various types. However, such stencils have not, to the knowledge of the applicant, been used with saturators. Instead, stencils have typically been used with applicators which apply liquid to the surface of a web without substantial impregnation. Examples of such applicators are spray devices (Smith U.S. Pat. No. 3,088,859); extruders (Sorg U.S. Pat. No. 2,904,448); roller applicators (Holdsworth U.S. Pat. No. 2,056,274); and spreaders (Hannington U.S. Pat. No. 1,546,834). Such applicators differ significantly from saturators in that they apply a liquid to the surface of the substrate without specific pressure to force the applied liquid into the interslices of the substrate and therefore do not provide deep impregnation as does a saturator.